The present invention relates to a driving device which controls a motor by a sensor-less vector control system without using any magnetic pole positional sensor.
Heretofore, in a case where a permanent magnet synchronous motor is operated by a sensor-less vector control, it is necessary to estimate a magnetic pole position of a rotor without using any sensor. As an estimating method, there are a method based on a fundamental wave of an induced electromotive voltage and a method based on a high harmonic wave, but the method based on the fundamental wave can only be applied to a medium to high speed region. That is, the induced electromotive voltage is small in a stop position and a low-speed region, it is difficult to detect the voltage, and a high-frequency voltage or current is injected to generate the high harmonic wave of the induced electromotive voltage and estimate the magnetic pole position. Therefore, in a sensor-less system for a high speed, the fundamental wave of the induced electromotive voltage is utilized. In a sensor-less system for a low speed, there is utilized a method of injecting the harmonic.
In an actual vector control, with respect to a d-q axis which the magnetic pole position of the rotor of the synchronous motor is a rotary position at a real angle θd, a dc-qc axis is supposed in which an estimated angle θdc is obtained in the control system. An axial error Δθ between the axis is estimated and calculated. So as to set this axial error Δθ to zero, the estimated magnetic pole position is feed back and corrected, and this allows an actual magnetic pole position to meet a controlled magnetic pole position.
According to such a vector control, it is possible to ideally control a torque generated in the motor by an inverter in accordance with load conditions, and it is possible to realize a control of the revolution speed of motor with high efficiency and precision. Since there is not any sensor-less vector control system that is usable at low to high speeds, however, there is proposed a method and the like in which, for example, after the motor is started under a constant V/F control without the necessity of detecting the magnetic pole position, the control is shifted to the vector control using a preset initial magnetic pole position at a predetermined revolution speed (see, e.g., Japanese Patent Application Laid-Open No. 2004-48886).
Moreover, there is also a system in which after starting the motor, the sensor-less vector control for the low speed is executed, and the control is shifted to the sensor-less vector control for the high speed. In this case, there is proposed the use of the magnetic pole position obtained by weighted-averaging the positions detected for the low and high speeds in the vicinity of the switching between the low speed and the high speed (see, e.g., Japanese Patent No. 3612636).
Thus, there are proposed various control systems after the motor starts until the motor reaches the predetermined revolution speed at which the detection of the magnetic pole position (estimation of the magnetic pole position using the induced electromotive voltage in the sensor-less vector control for the high speed) is possible in a case where the sensor-less vector control for the high speed is performed. However, if an axial error is large between the actual magnetic pole position immediately after the control has shifted to the sensor-less vector control and the initially set magnetic pole position, there rises a danger that the motor runs out of step and fails in starting in the feedback control.
Moreover, even in the system in which the sensor-less vector control for the low speed shifts to the sensor-less vector control for the high speed as described above, the motor easily runs out of step immediately after the shifting. Furthermore, in a case where a load torque is larger or, for example, a case where a difference between a high pressure and a low pressure of a refrigerant circuit becomes large in the motor for a compressor to thereby increase fluctuations of the load torque, the motor easily runs out of step especially during the shift to the sensor-less vector control described above.